# Example This shows a "plain" use of Remark.js fails in rendering by MathJax. This works: $$\sum_{a_i\in A_j}f(a_i)$$ but this fails: $$\sum_{i=1}^n \lVert a_i \Vert_\infty$$ This is because the source: ``` $$\sum_{i=1}^n \lVert a_i \Vert_\infty$$ ``` is converted as: ``` $$\sum<em>{i=1}^n \lVert a_i \Vert</em>\infty$$ ```